I. Field of the Invention
This invention relates to a fluid valve which allows the downward flow of drilling fluids but closes when pumps are off to retain the fluid within the drill pipe and, in particular, to a mud saver valve which includes a check valve to bleed off downhole pressure with an increased flow area to prevent blowout of the closure plug under extreme or sudden fluid pressure.
II. Description of the Prior Art
In the drilling of oil and gas wells, it is common practice to insert in the drill string between the kelly and the drill pipe a valve to retain mud in the kelly when the drill string is detached. The advantages of mud saver valves include saved mud cost, decreased chances of pollution, and increased safety to rig personnel. Most of the past known mud saver valves include a piston having an axial throughbore biased upwardly within the valve housing by a spring. When closed, the piston engages a closure plug which blocks mud flow through the piston and valve. As the surface pumps pump drilling mud through the drill string, the fluid pressure acts against the top of the piston and the force of the spring to open the fluid passageway through the valve. When mud flow is interrupted the spring forces the piston against the closure plug to close the passageway.
The closure plug is removable from the mud saver valve to permit the running of wireline tools through the drill string. Typically, the closure plug is provided with a spear head such that an overshot may be run to grasp the plug for removal. Many of the past known closure plugs also include a check valve to bleed off excess downhole fluid pressure when the valve is closed. However, such past known check valves utilize a simple ball valve having a small fluid capacity. In the event of sudden or extreme downhole fluid pressure, the ball check valves are not capable of accommodating the increased pressure and flow. Since the closure plugs are typically retained within the valve by shear screws, the extreme pressure has been known to blow the closure plug out of the valve resulting in mud flow to the rig surface and a potentially dangerous situation.
In addition, the ball-type check valves of the past known mud saver valves have a tendency to clog with drilling mud because of their limited fluid capacity. Heavier muds tend to accumulate within the flow passageway and on the movable ball and stem of the check valve. Since closure of the check valve is dependent upon cooperation of the ball with the valve seal, the accumulation of mud may prevent the check valve from closing when pressure has been substantially equalized.